The invention relates to a capacitive noncontact gauging system for measuring the surface characteristics of a work piece. The system consists of a multiplicity of probes which are placed adjacent to the grounded work piece. The probes and work piece form, in effect, a variable capacitor whose capacitance varies with the spacing between the work piece and the probe. Various schemes have been used to measure this capacitance and produce an electric signal which is indicative of the probe-to-work piece distance. One method of doing this is the frequency modulation method, an example of which is shown in U.S. Pat. No. 3,716,782 of J. J. Henry, or U.S. Pat. No. 3,775,679 to R. C. Abbe. In the frequency modulation method the change in capacitance is used to modify the frequency of an oscillator, which change in frequency is used to produce the output distance indicative signal. A second approach is an amplitude sensitive system in which a capacitor of known capacitance and the probe-to-work piece capacitance act as a voltage divider. The ratio of voltage across both capacitances varies as the variable capacitance varies. In the amplitude sensitive system, the output from the voltage divider is processed and normally transformed into an indication of the distance between the probe and the work piece.
Although many of the inventive concepts of the present invention are applicable to frequency modulation systems, the present invention is an amplitude sensitive system. The present system presents many improvements over other amplitude sensitive systems and frequency modulation systems. One of the advantages is in the elimination of parallel parts. Many of the prior systems required an oscillator and signal processing circuitry for each probe. The present invention, by contrast, has a single oscillator and a single signal processing circuit for a large multiplicity of probes. The present invention contemplates using 200 or more probes with a single oscillator and a single processing circuit.
An additional advantage of the present system is in its temperature stability. The changes in capacitance between the probes and the work pieces are generally very small which makes capacitive measuring systems very susceptible to error from temperature changes and stray capacitance. Small changes in temperature have been found to cause large variations in many systems. In the present invention, a very low input capacitance and high impedance unity gain amplifier is used to improve temperature stability. The amplifier of the present invention has been found to be accurate to 0.1% over a temperature range of 35.degree. to 150.degree. F.
The present invention, because of its temperature stability and other superior design characteristics, is able to measure very small changes in distance. Distances of a few millionths of an inch can be detected. This great precision is required when measuring many surface characteristics such as straightness or conformance to a contour. The present gauging machine finds ready use for determining the flatness of machine molded, stamped, cast or fabricated parts, the straightness of cylinder walls or holes, and the conformance to specified contours of such complex parts as step shafts, air foils, and turbine blades.
A further advantage of the present invention is its rapid measuring ability. Although the speed will vary with the number of probes used, it has been found that with 240 probes the present invention can usually produce an indication of the conformance of a work piece at 240 positions within 5 seconds. This rapidly increases the precision of measurement instruments because the surface contour of the work piece can be sampled at such a large number of points. Further, this rapidity enables testing equipment to sample large numbers of work pieces in a relatively short amount of operating time.
The present system has a further advantage in that the large number of probes can be arranged in a linear array or in a grid array or along a winding contour or in most any other array. The linear array, however, produces the most easily understood two dimensional display of surface contour. Because of linear array of the present invention can measure 200 or more points along a single line of contact of the work piece, the system can produce a very detailed two dimensional, cross section-like view of the surface along the line of closest proximity between the work piece and the line of probes. The work piece may be shifted or rotated, to view other line segments along its surface.